The present invention relates to an image forming apparatus having therein a recording material housing means which houses recording materials, an image forming means which forms an image on a recording material that is in transit, and a conveyance means which conveys a recording material from the recording material housing means to the image forming means along a conveyance path.
In the image forming apparatus such as a copying machine of an electrophotographic type and a printer, a recording material housed in a recording material housing means is conveyed by a conveyance means, and image forming is conducted by an image forming means on the recording material thus conveyed. In recent years, there has been a strong demand for improvement in the number of sheets for image forming per unit time (productivity), and speedup of the conveyance speed (hereinafter referred to as "process speed") for the recording material in the case of forming images with an image forming means is now being sought, accordingly.
For making a process speed higher, however, it is necessary to increase strength of a housing of an image forming apparatus, which results in a cost increase. Further, when a process speed is higher, a time lag in the conveyance timing makes misregistration of an image on a recording material more likely to occur. Since the frictional resistance between a recording material and a conveyance path is increased and an extent of slippage between a recording material and a conveying member is increased by the higher process speed, in particular, a time lag in the conveyance timing for the recording material and an extent of sheet skewing are increased. In addition, when complying with many types of recording materials, a time lag in conveyance timing and an extent of sheet skewing vary depending on the frictional force between a recording material and a conveyance path and that between the recording material and a conveying member These frictional forces are different from each other. Furthermore, when plural means for loading recording materials are provided for complying with many types of recording materials, a length of the conveyance path varies depending on each recording material loading means, which makes a time lag in conveyance timing and an extent of sheet skewing to be different. When increasing the number of images formed in the unit time by gathering the process speed, therefore, it is necessary to enhance accuracy of conveyance of recording sheets for forming images with high quality.
For enhancing productivity of images, on the other hand, there is also a method to narrow intervals for conveying recording materials, in addition to gathering of the process speed. However, even when the conveyance intervals are narrowed, it is still necessary to enhance accuracy of conveyance for recording sheets, because there is a high possibility, if the conveyance intervals are narrowed, that a trailing edge portion of a preceding recording sheet and a leading edge portion of a succeeding recording sheet are superposed on each other and that a paper jam will be caused when the conveyance timing for recording sheets is different between a preceding recording sheet and a succeeding recording sheet.
Namely, regardless of whichever method is employed, it is necessary, for raising productivity, to enhance the accuracy of conveyance of recording sheets. After studying the location where a time lag in conveyance timing for recording sheets and sheet skewing tend to be caused firstly, it was found that the location was a sheet feeding section where a recording sheet is taken out of a recording sheet loading means, and that time lag and sheet skewing were caused by frictional force between recording materials. It was also found that the location which tends to cause a time lag and sheet skewing secondly was a portion in the conveyance path where there is a curve and that time lag and sheet skewing were caused by frictional force between the recording material and the conveyance path. The time lag and skewing caused in the aforesaid locations can be corrected relatively easily if they are corrected in the early stage, but when they are not corrected in the early stage, they are increased by the conveyance thereafter to become difficult to be corrected easily, resulting in paper jams which tend to stop an apparatus.